MRAM Cells Including Coupled Free Ferromagnetic Layers for Stabilization

ABSTRACT

A free ferromagnetic data storage layer of an MRAM cell is coupled to a free ferromagnetic stabilization layer, which stabilization layer is directly electrically coupled to a contact electrode, on one side, and is separated from the free ferromagnetic data storage layer, on an opposite side, by a spacer layer. The spacer layer provides for the coupling between the two free layers, which coupling is one of: a ferromagnetic coupling and an antiferromagnetic coupling.

BACKGROUND

Magnetoresistive random access memory (MRAM) typically employs an arrayof magnetic storage elements, or cells, which are each located at, ornear, an intersection, or crossing, of a corresponding word line with acorresponding bit line. Those skilled in the art know that spin transfercan be used as an alternative to, or in addition to, an externalmagnetic field in programming current perpendicular to plane (CPP)configurations MRAM cells, which may be either of the magnetic tunneljunction (MTJ) type or of the spin valve (SV) type. When aspin-polarized write current passes through a data storage layer of thecell, which is a free ferromagnetic layer, a portion of the spin angularmomentum of the electrons incident on the data storage layer istransferred to the data storage layer. A spin transfer effect, that iscaused by conduction electrons traveling from a pinned ferromagneticlayer of the cell to the data storage layer, switches the magnetizationorientation of the data storage layer from a direction that is oppositeto that of the magnetization orientation of the pinned layer, to adirection that coincides with that of the magnetization orientation ofthe pinned layer, for example, to program, or write, a logical “0” tothe cell; and, a spin transfer effect that is caused by conductionelectrons traveling in the opposite direction, switches themagnetization orientation of the data storage layer back to thedirection that is opposite to that of the magnetization orientation ofthe pinned layer, for example, to write a logical “1” to the cell.

In some MRAM arrays, data storage layers may be susceptible to aninadvertent switching, for example, caused by thermally induced latticevibration. This thermal instability of the storage layers may be due toa reduction in the size and/or magnetization thereof. Furthermore, asignificant amount of Joule heating may be generated by a write current,and those cells, which are adjacent to one being written, particularlyin ultra high density MRAM arrays, may be inadvertently switched due tothe heating. Thus, there is a need for MRAM cell configurations thatprovide for enhanced thermal stability.

BRIEF SUMMARY

A first free ferromagnetic layer of an MRAM cell, which functions fordata storage, is separated, on one side, by a first spacer layer, from apinned ferromagnetic layer, and is separated, on an opposite side, by asecond spacer layer, from a second free ferromagnetic layer, which isdirectly electrically coupled to a contact electrode, and which acts tostabilize the cell, via coupling across the second spacer layer. Thecoupling is one of a ferromagnetic coupling and an antiferromagneticcoupling, and has a strength that is greater than a coercivity of thesecond free ferromagnetic layer. When a writing current is applied tothe cell, a magnetization orientation of the first free ferromagneticlayer is reversed by a spin transfer effect of the current, and, then, amagnetization orientation of the second free ferromagnetic layer isreversed, via the coupling between the first and second free layers, tostabilize the new magnetization orientation of the data storage layer,just programmed by the writing current. The second spacer layer mayinclude one or more sub-layers and may further provide for spindepolarization.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of thedisclosure and therefore do not limit the scope. The drawings are not toscale (unless so stated) and are intended for use in conjunction withthe explanations in the following detailed description. Embodiments ofthe disclosure will hereinafter be described in conjunction with theappended drawings, wherein like numerals denote like elements.

FIG. 1 is a schematic showing a basic configuration of an MRAM cell,according to some embodiments.

FIGS. 2A-C are schematics showing a sequence of events associated withthe application of a write current to the cell of FIG. 1.

FIGS. 3A-C are schematics showing various configurations of an MRAMcell, according to alternate embodiments.

DETAILED DESCRIPTION

The following detailed description is exemplary in nature and is notintended to limit the scope, applicability, or configuration in any way.Rather, the following description provides practical illustrations forimplementing exemplary embodiments.

FIG. 1 is a schematic showing a basic configuration of an MRAM cell 100,according to some embodiments. FIG. 1 illustrates cell 100 including apinned layer 112, a free ferromagnetic data storage layer 111, a firstspacer layer 121, which extends between data storage layer 111 andpinned layer 112, a free ferromagnetic stabilization layer 113, which isdirectly electrically coupled to a first electrode contact layer 101,and a second spacer layer 131, which extends alongside stabilizationlayer 113, opposite first electrode contact layer 101, and betweenstabilization layer 113 and data storage layer 111. First spacer layer121 is nonmagnetic and may be either conductive, for example, formedfrom metallic materials, such as Au, Ag, and Cu, or insulative, forexample, formed from oxide and semiconductor barriers, such as AlO, TiOand MgO. FIG. 1 further illustrates pinned layer 112 of cell 100, whichmay either be a single layer or a synthetic antiferromagnetic coupledstructure (SAF), being pinned by an adjacent antiferromagnetic pinninglayer 110, which is directly electrically coupled to a second electrodecontact layer 103. Those skilled in the art will appreciate that,although not shown, cell 100 is electrically coupled, via electrodecontacts 101 and 103, to an intersection of a corresponding word lineand bit line of an MRAM array, which may include on the order of 1,000,or more, intersecting word and bit lines and corresponding MRAM cells.

According to embodiments of the present disclosure, second spacer layer131 is conductive and provides either ferromagnetic coupling orantiferromagnetic coupling between data storage layer 111 andstabilization layer 113, wherein a strength of the coupling is greaterthan a coercivity of the stabilization layer 113; when a magnetizationorientation of data storage layer 111 is switched, or re-programmed, themagnetization orientation of stabilization layer 113 will follow, aswill be further described in conjunction with FIG. 2C. The couplingbetween free layers 111, 113, whether ferromagnetic orantiferromagnetic, stabilizes cell 100 against, for example, inadvertentheat-induced switching. Free layers 111, 113 may be formed fromtransition metals, such as Ni, Co and Fe, alloys thereof, for example,NiFe and CoFe, or ternary alloys, such as CoFeB. According to anexemplary embodiment, stabilization layer 113 has a coercivity in arange from approximately 50 Oe to approximately 200 Oe, and isapproximately 4 nm thick, and data storage layer 111 is approximately 3nm thick. A thickness of second spacer layer 131 may be betweenapproximately 2 nm and approximately 20 nm, and second spacer layer 131may further provide for spin depolarization. Second spacer layer 131 maybe formed from a conductive and antiferromagnetic material, such asFeMn, RhMn, FeRhMn, IrMn, PtMn, PdMn, PtPdMn, NiMn, CrMn or CrPtMn, orfrom a conductive and paramagnetic material, for example, formed bydoping nonmagnetic metals, or alloys, such as Al, Cu, Ag and Pt, withmagnetic atoms of, for example, Fe, Co, Ni, Cr or Mn. According to yetfurther embodiments, second spacer layer 131 is formed from a conductiveand non-magnetic material, such as Al, Cu, Pt, Ag or Ru.

FIGS. 2A-C are schematics showing a sequence of events associated withthe application of a write current to cell 100. FIG. 2A illustrates cell100 in which data storage layer 111 has been programmed such that themagnetization orientation thereof is aligned with that of pinned layer112. FIG. 2A further illustrates second spacer layer 131 providing forantiferromagnetic coupling between stabilization layer 113 and datastorage layer 111, in order to stabilize the magnetization orientationof layer 111, until a write current is applied to cell 100 in order tore-program cell 100. FIG. 2B schematically illustrates the applied writecurrent with an arrow W. As previously described, cell 100, iselectrically coupled, via contacts 101 and 103, between a word line andbit line of an MRAM array such that a voltage potential, between thelines, drives write current W. FIG. 2B further illustrates, data storagelayer 111 having been switched, or reprogrammed, to an oppositemagnetization orientation, via a spin transfer effect of write currentW. Once the magnetization orientation of layer 111 has been reversed,antiferromagnetic coupling between layer 111 and stabilization layer113, will cause the magnetization orientation of layer 113 to reverse,as illustrated in FIG. 2C. In FIG. 2C, cell 100 is re-programmed andstabilized by the antiferromagnetic coupling between free layers 111 and113.

The antiferromagnetic coupling between layers 111 and 113, which isillustrated by FIGS. 2A-C, may be achieved by magnetostatic coupling.Magnetostatic coupling, that has a significant coupling strength, canexist across second spacer layer 131, which has a thickness of overapproximately 20 nm, and may be independent of a material that formsspacer layer 131. Alternately, ferromagnetic coupling may be achieved byNeel coupling (a.k.a. orange-peel coupling), which arises from interfaceroughness, or via RKKY interaction. RKKY coupling is oscillatory and canbe either ferromagnetic or antiferromagnetic, depending upon a thicknessof second spacer layer 131. The oscillation period is typically about0.5 nm and also depends upon the materials from which spacer layer 131,and free layers 111, 113 are formed. By carefully choosing the spacerthickness and material, and by engineering the interface roughness tocancel out the magnetostatic coupling, a net ferromagnetic coupling canbe achieved. A thickness of second spacer layer 131 that provides forferromagnetic coupling may be less than approximately 10 nm.

FIGS. 3A-C are schematics showing various configurations of an MRAM cell300A, 300B and 300C, respectively, according to alternate embodiments.Each of cells 300A-C is similar to cell 100 in that each includeselectrode contact layers 101, 103, pinned layer 112, which is pinned bylayer 110, first spacer layer 121, which separates pinned layer 112 fromfree ferromagnetic data storage layer 111, and free ferromagneticstabilization layer 113. In contrast to cell 100, each of cells 300A-Cincludes a multi-layer conductive spacer layer 231A, 231B and 231C,respectively.

FIG. 3A illustrates conductive spacer layer 231A of cell 300A includinga first sub-layer 21 and a second sub-layer 31, wherein second sub-layer31 is directly adjacent data storage layer 111 and has a thickness thatis less than that of first sub-layer 21. FIG. 3B illustrates conductivespacer layer 231B of cell 300B also including first and secondsub-layers 21, 31, except that the arrangement thereof is reversed fromthat for cell 300A. According to some embodiments, first sub-layer 21 isnon-magnetic and may have a thickness of approximately 4 nm, and secondsub-layer 31 may either be antiferromagnetic or paramagnetic, and mayhave a thickness of approximately 2 nm. According to those embodimentsin which second sub-layer 31 is paramagnetic, second sub-layer 31 may beformed by doping first sub-layer 21 with a magnetic material. FIG. 3Cillustrates conductive spacer layer 231C of cell 300C including a firstsub-layer 41, a second sub-layer 42 and a third sub-layer 43; firstsub-layer 41 is directly adjacent data storage layer 111, secondsub-layer 42 extends between first and third sub-layers 41, 43, andthird sub-layer 43 is directly adjacent stabilization layer 113. FIG. 3Cfurther illustrates first and third sub-layers 41, 43 having a thicknessthat is less than that of second sub-layer 42. According to someembodiments, second sub-layer 42 is non-magnetic and may have athickness of approximately 4 nm, and first and third sub-layers 41, 43are each either antiferromagnetic or paramagnetic and may each have athickness of approximately 2 nm. According to some additionalembodiments, first and third sub-layers 41, 43 are each non-magnetic andsecond sub-layer may be either antiferromagnetic or paramagnetic.Multi-layer conductive spacer layers 231A, 231B, 231C may beparticularly suited to depolarize conduction electron spin orientationin order to prevent spin transfer in free ferromagnetic stabilizationlayer 113. It should be noted that material matching at the interfacesof the sub-layers for these multi-layer spacer layer embodiments isimportant to prevent interlayer mixing of atoms, due to thermaldiffusion. Furthermore, the thicknesses of the sub-layers should beadjusted to achieve the desired magnetic coupling strength,spin-depolarization efficiency and diffusion-blocking ability. Examplesof some suitable antiferromagnetic materials, which may be incorporatedby multi-layer conductive spacer layers 231A, 231B, 231C, include,without limitation, FeMn, RhMn, FeRhMn, IrMn, PtMn, PdMn, NiMn, CrMn andCrPtMn. Examples of some suitable conductive non-magnetic materials forspacer layers 231A, 231B, 231C include, without limitation, Al, Cu, Pt,Ag, Au, Ru and alloys thereof; and these non-magnetic materials may bedoped with magnetic atoms such as Fe, Co, Ni, Cr or Mn, to form suitableparamagnetic materials for layers 231A, 231B, 231C.

In the foregoing detailed description, embodiments of the disclosurehave been described. These implementations, as well as others, arewithin the scope of the appended claims.

1. A memory element semiconductor stack comprising: a free ferromagneticdata storage layer having a first thickness; a free ferromagneticstabilization layer having a second thickness greater than the firstthickness; and a multi-layered coupling layer disposed between andadapted to ferromagnetically couple the data storage layer and thestabilization layer.
 2. The semiconductor stack of claim 1, in which anoverall strength of the ferromagnetic coupling between the data storagelayer and the stabilization layer is greater than an overall coercivityof the stabilization layer.
 3. The semiconductor stack of claim 1, inwhich the multi-layered coupling layer has at least one non-magneticlayer and at least one magnetic layer.
 4. The semiconductor stack ofclaim 1, further comprising a pinned layer having a fixed magneticorientation, and a spacer layer disposed between the pinned layer andthe data storage layer, the stack storing a first data state responsiveto the free layer having a magnetic orientation that coincides with thefixed magnetic orientation of the pinned layer, the stack storing asecond data state responsive to the free layer having a magneticorientation opposite the fixed magnetic orientation of the pinned layer.4. The semiconductor stack of claim 1, in which the free ferromagneticdata storage layer is formed from a transition metal.
 5. Thesemiconductor stack of claim 1, in which the free ferromagneticstabilization layer is formed from a transition metal.
 6. Thesemiconductor stack of claim 1, in which the multi-layered couplinglayer ferromagnetically couples the data storage layer and stabilizationlayer in an anti-parallel magnetic relationship.
 7. The semiconductorstack of claim 1, in which the multi-layered coupling layer comprises afirst sub-layer and a second sub-layer, the second sub-layercontactingly engaging the data storage layer and having a thickness lessthan that of the first sub-layer.
 8. The semiconductor stack of claim 1,in which the multi-layered coupling layer comprises a first sub-layerand a second sub-layer, the first sub-layer contactingly engaging thedata storage layer and having a thickness greater than that of thesecond sub-layer.
 9. The semiconductor stack of claim 1, in which themulti-layered coupling layer comprises respective first, second andthird sub-layers, the second layer disposed between and having a greaterthickness than the first and third sub-layers.
 10. The semiconductorstack of claim 1, in which the multi-layered coupling layer comprises afirst sub-layer which contactingly engages the data storage layer and asecond sub-layer which contactingly engages the stabilization layer, aselected one of the first or second sub-layers formed of a non-magneticmaterial and a remaining one of the first or second sub-layers formed ofa magnetic material.
 11. The semiconductor stack of claim 1, furthercomprising a conductive electrode contactingly engaging a top surface ofthe stabilization layer, and a conductive control line connected to theconductive electrode.
 12. A data storage device comprising a cross-pointarray of memory cells arranged into rows and columns and interconnectedvia respective bit and word lines, each of the memory cells comprising afree ferromagnetic data storage layer, a free ferromagneticstabilization layer, and a multi-layered coupling layer disposed betweenand adapted to ferromagnetically couple the data storage layer and thestabilization layer.
 13. The data storage device of claim 12, in whichan overall strength of the ferromagnetic coupling between the datastorage layer and the stabilization layer is greater than an overallcoercivity of the stabilization layer.
 14. The data storage device ofclaim 12, in which the multi-layered coupling layer has at least onenon-magnetic layer and at least one magnetic layer.
 15. The data storagedevice of claim 12, in which each of the memory cells further comprisesa pinned layer having a fixed magnetic orientation, and a spacer layerdisposed between the pinned layer and the data storage layer, the cellstoring a first data state responsive to the free layer having amagnetic orientation that coincides with the fixed magnetic orientationof the pinned layer, the cell storing a second data state responsive tothe free layer having a magnetic orientation opposite the fixed magneticorientation of the pinned layer.
 16. A method of forming a memory cell,comprising: forming a free ferromagnetic data storage layer having afirst thickness; forming a multi-layered coupling layer on the datastorage layer; and forming a free ferromagnetic stabilization layer onthe coupling layer having a second thickness greater than the firstthickness, the coupling layer adapted to ferromagnetically couple thedata storage layer and the stabilization layer.
 17. The method of claim16, in which the coupling layer comprises a first sub-layer ofnon-magnetic material and a second sub-layer of magnetic material. 18.The method of claim 17, in which the first and second second sub-layersare formed by depositing a layer of non-magnetic material with aselected overall thickness nominally corresponding to a combinedthickness of the first and second sub-layers, and doping a portion ofsaid deposited layer with a magnetic material.
 19. The method of claim16, further comprising forming a conductive top electrode on thestabilization layer.
 20. The method of claim 16, further comprisingprior steps of forming a pinning layer on a lower electrode layer,forming a pinned layer on the pinning layer, and forming a spacer layeron the pinned layer, wherein the data storage layer is subsequentlyformed on the spacer layer.